RIIIS/J mice make low antibody responses to several polysaccharide antigens of bacterial origin. They have low levels of serum IgM and IgG3, and high levels of serum IgG2a and IgG2b. Low serum IgM and IgG3 levels appear to be due to a low frequency of Ly-1 B cells. RIIIS/J mice treated with bacterial lipopolysaccharide (LPS) produce a low anti-bromelain-mouse-erythrocyte (anti-BrMRBC) antibody response, but a normal antibody response against mouse transferrin. These data are consistent with the fact that Ly-1 B cells contain the precursors of B lymphocytes secreting anti-BrMRBC antibody. RIIIS/J mice should be valuable in studies on the development of the Ly-1 B cell lineage.